1. Technical Field
The present application relates to a terahertz electromagnetic wave generator, a terahertz spectrometer, and a method of generating a terahertz electromagnetic wave.
2. Description of the Related Art
In this specification, the “terahertz electromagnetic wave” will refer herein to an electromagnetic wave, of which the frequency falls within the range of 0.1 THz to 100 THz. 1 THz (terahertz) is 1×1012 (=the twelfth power of 10) Hz. Terahertz electromagnetic waves are now used in various fields including security, medical treatments, and nondestructive tests on electronic parts. Since there are excitation, vibration and rotation modes of various electronic materials, organic molecules and gas molecules in the terahertz electromagnetic wave frequency range, people have proposed that a terahertz electromagnetic wave be used as a sort of “fingerprint” to recognize a given material. On top of that, since a terahertz electromagnetic wave is safer than an X ray or any of various other electromagnetic waves, the terahertz electromagnetic wave can be used to make a medical diagnosis without doing harm on the body of a human subject.
As disclosed in Nature Mater. 1, 26, (2002), a photoconductor or a nonlinear optical crystal is used as a conventional terahertz electromagnetic wave generator. In any of those elements, a terahertz electromagnetic wave is generated by irradiating the element with a laser beam, of which the pulse width falls within the range of a few femtoseconds to several hundred femtoseconds (and which will be hereinafter referred to as a “femtosecond laser beam”). 1 femtosecond is 1×10−15 (=the minus fifteenth power of 10) seconds. In a vacuum, an electromagnetic wave travels approximately 300 nm in one femtosecond.
Such a terahertz electromagnetic wave is generated by taking advantage of a so-called “dipole radiation” phenomenon in classical electromagnetism. That is to say, a variation in electric polarization or current in accelerated motion with time generates an electromagnetic wave at a frequency corresponding to the rate of that variation. Since a variation in polarization or current is induced in a few femtoseconds to several hundred femtoseconds (which depends on the pulse width of the laser beam) by being irradiated with a femtosecond laser beam, the electromagnetic wave generated by dipole radiation has a frequency falling within the terahertz range.